March 4, 2013

Patterson Glacier Retreat, Southeast Alaska

Posted by Mauri Pelto

Patterson Glacier is a western outlet of the Stikine Icefield in southeast Alaska. The glacier does not reach tidewater it is 23 km long and has an area of about 100 km2. At the end of the 19th century, the terminus flowed both the east and the west, and was slowly advancing. William O. Field visited the glacier in 1941 and 1948 and found a 1.3 km retreat since the late 19th century. By 1979 the USGS noted a further 1 km of retreat. I saw the glacier enroute to the Juneau Icefield in 1984 and the lake at the terminus was still rather small. I talked to William O. Field right after that field season, he was convinced that all Stikine Icefield glaciers would be retreating more and we discussed Baird Glacier just to the north because it has not retreated yet, though it is showing signs now. In this post we examine the changes between 1985 and 2011 using Landsat imagery. Since 1984 frequent satellite imagery allows identification of the equilibrium line altitude (ELA), the snowline at the end of summer where accumulation equals ablation that year. The average ELA has been 1150 m, which given that the main accumulation zone ends at 1350 m is too high to maintain equilibrium. The green arrows indicate flow directions in the Google Earth image below. The yellow arrow the 1985 terminus, the red arrow the 2011 terminus, the blue arrows the snowline and the pink arrow a peninsula where a prominent debris band is located in 2011, that is also marked by the letter D. The debris band represents an avalanche that descended onto the glacier not too long before 1985. Glacier velocity is faster in the center and this fairly round debris will quickly become drawn down glacier faster in the center than at its edge, this had not notably occurred yet in 1985. patterson glacier ge A comparison of the 1985 Landsat image of Patterson Glacier and 2011 Landsat image indicate that the debris band (D) has shifted 3000 meters down glacier in 27 years, indicating approximately 110 meters per year velocity for the north side of the Patterson Glacier valley tongue. The terminus has retreated from the yellow to the red arrow a distance of 1200 meters, a rate of 45 m/year. The terminus lake is 2 km long in 2011. The 2011 terminus is quite narrow indicating the glacier maybe reaching the eastern end of this lake basin. A close up of the terminus area in Google Earth indicates the same features, orange arrows indicate the east and west flow at the terminus that was observed in the late 19th century. This glacier is responding very similarly to Speel Glacier, Great Glacierand Norris Glacier of the Juneau Icefield
patterson galcier 1985

patterson glacier 2011

patterson terminus